Step Down Transformer (what's my best option?)

Hero999 said:

Me included. Earthing a scope always has made sense to me and the more I read Nigel's arguments against it the more I believe I'm right.

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No problem - everyone is entitled to their opinion - not earthing a scope keeps me (and thousands of others) safer, so we'll continue to do so.

As far as I know, electrical companies in Canada use a single wire for each of the 3 phases as the extremely high voltage transmission lines and the earth as their return.

I think I have "a 3-wire, single-phase, mid-point neutral system" (120V-neutral-120V) from the transformer, and I think my neutral is grounded to cold water pipes in my home.

Wikipedia knows wuite a bit about single wire earth return.
https://en.wikipedia.org/wiki/SWGR

It's normally used for single phase distribution not three phase. I suspect that only three wires are used in very high voltage transmission and a transfromer with a star winding provides all the local consumers, each street having it's own 120V-0-120V transformer.

Hero999 said:

I suspect that only three wires are used in very high voltage transmission.

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As I said above! - there's no need for a neutral as long as the phases are balanced, and Delta configuration doesn't even have a neutral.

I agree, but the phases don't even need to be balanced either way a neutral isn't needed. The only time a neutral is used is when a three phase supply is litterly supplying three single phase loads, each having a neutral connection.

Hero999 said:

I agree, but the phases don't even need to be balanced either way a neutral isn't needed. The only time a neutral is used is when a three phase supply is litterly supplying three single phase loads, each having a neutral connection.

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Yes, for pure 3 phase use a neutral isn't needed at all, but for single phase distribution from sub-station to users a neutral is obviously essential - but by carefully 'balancing' the load across the three phases you can use a cheaper cable for the neutral.

This thread seems to have deviated quite a bit from its original course.
Let me now split it into two different directions.

Earthed/Un-earthed Scopes
Nigel is happy with his un-earthed scope, many others, myself included, much prefer to keep them earthed for reasons of electrical safety. That is OK.

But consider, chassis of the un-earthed scope will probably sit at about half supply voltage (120v in the UK) due to the stray capacitance in the mains transformer etc. This 120v will have a high impedance and is most unlikely to cause harm to any healthy person.
However, one of the big concerns in the industry is ESD, Electro Static Discharge. With the smaller structures within integrated circuits, it is possible to electrically overstress the components and damage them, due to static build up on personnel, general items in the work area, and things like un-earthed soldering irons and test equipment. 120v from the un-earthed scope can easily zap CMOS ICs.
Is there any emphasis placed on ESD precautions in the consumer electronics repair industry?


Electrical Distribution Systems
The high voltage systems are three phase, and a neutral is not required, in a balanced three phase system, the neutral current should be zero.
At the local distribution level, from the local transformer, there are three phase conductors and a neutral conductor. The neutral is earthed, I believe near to the transformer.
Businesses that need a higher power will be connected to the three phases and neutral, a domestic user will be connected to one of the phases and neutral.
I have seen local distribution systems which use just one wire!
The return is done via the earth.
This was in Iceland, have a look at the attached picture, this is the power feed to a clifftop lighthouse at Dyrholaey on the south coast.
Curious stuff, I wonder what the load regulation is like?

JimB

JimB said:

But consider, chassis of the un-earthed scope will probably sit at about half supply voltage (120v in the UK) due to the stray capacitance in the mains transformer etc. This 120v will have a high impedance and is most unlikely to cause harm to any healthy person.

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Alright, now I undersatnd where that 120V comes from. My concern about an unearthed scope is that someone might try to measure an eath-bonded high voltage which would render the case live otherwise if it was earthed a fuse would blow. This is why you should power what you're testing from an isolation transformer, in other words you should always isolate the appliance under test from earth not the test equipment.

JimB said:

However, one of the big concerns in the industry is ESD, Electro Static Discharge. With the smaller structures within integrated circuits, it is possible to electrically overstress the components and damage them, due to static build up on personnel, general items in the work area, and things like un-earthed soldering irons and test equipment. 120v from the un-earthed scope can easily zap CMOS ICs.

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I hadn't though about the ESD implications.

JimB said:

Electrical Distribution Systems
The high voltage systems are three phase, and a neutral is not required, in a balanced three phase system, the neutral current should be zero.
At the local distribution level, from the local transformer, there are three phase conductors and a neutral conductor. The neutral is earthed, I believe near to the transformer.
Businesses that need a higher power will be connected to the three phases and neutral, a domestic user will be connected to one of the phases and neutral.

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In Europe, yes but I wonder what they do in the US, where they power each house from a three wire single phase system. I assume the power each centre tapped transformer from a different phase.

JimB said:

I have seen local distribution systems which use just one wire!
The return is done via the earth.
This was in Iceland, have a look at the attached picture, this is the power feed to a clifftop lighthouse at Dyrholaey on the south coast.
Curious stuff, I wonder what the load regulation is like?

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Perhaps being near the sea the ground has a high salt content, making ti a good conductor.

That lighthouse supply is bizarre, the return current actually flows through the earth, not a wire? I thought even a few metres of earth would have many hundreds of ohms resistance and would render the supply useless except for signals?

Dr.EM said:

That lighthouse supply is bizarre, the return current actually flows through the earth, not a wire? I thought even a few metres of earth would have many hundreds of ohms resistance and would render the supply useless except for signals?

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You obviously didn't click on the Wikipedia link a few posts earlier - give that a try!.

Even after reading it I don't think I understand it. I see it uses a higher voltage at around 19Kv to help overcome losses, but also talkes about 36mile runs? I mean, if I put a multimeter with one end in Bristol and one here in Swindon i'm sure it'd read right off the scale, way over 2M ohms? Or mabye it conducts better with HV and current flowing through it?

How many wires do you think they used in the Wild West for the telegraph systems? - just the one! - with the return through the ground.

I thought the Wikipedia article suggested a ground resistance value?, it will be fairly low - bear in mind there's a LOT! of it, all in parallel. The reason for the high voltage, as with all electrical distribution, is to reduce the current and lower the losses. It's only to your home at 120V or 230V, elsewhere it's much higher.

In this case the ground is highly conductive because it's saturated with salt water and they're probably using a high quality earth connection consisting of a very large metal plate buried deep underground, therefore resistance will be very low.

Hero999 said:

In this case the ground is highly conductive because it's saturated with salt water and they're probably using a high quality earth connection consisting of a very large metal plate buried deep underground, therefore resistance will be very low.

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Well the Wikipedia article you posted says:

A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the ground, and bonded to the transformer earth and tank. A good ground resistance is 5-10 ohms.

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I suppose so, I just find it wierd, I mean if you were to wrap a thin layer of platic over the earth rod it would completely break the circuit, so to imagine it travelling through many miles of earth without significant loss seems odd.

Telegraph systems don't carry much power do they? I can see this working for communications quite feasably, although in the wild west, the dry ground would probably be quite resistant, so a long pole driven down to moist soild must have been used.

BTW, if you were to touch a live mains in the upstairs of a building is there any chance of getting shocked? I mean you have a huge resistance to earth from there, I can't imagine 240v producing any current through about 2 metres of air and other non-conductive materials? For that matter, my kitchen floor is vinyl for instance, the resistance between the sides of one tile must be many Mohms, surely I can't carry a significant current to earth stading on them?

The best way I suppose is to use a mega, connect one probe to the earth wirw in a socket and hold the other wire.

One thing I've noticed is that some ceramics can have quite a low resistance at higher voltages because they often have iron embedded in them.

Dr.EM said:

I suppose so, I just find it wierd, I mean if you were to wrap a thin layer of platic over the earth rod it would completely break the circuit, so to imagine it travelling through many miles of earth without significant loss seems odd.

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If you cut the wire and wrap the end in plastic it wouldn't work either - where's the difference?.

Read the Wikipedia article, or read the quote above I posted from it! - ground resistance 5-10 ohms!.

Telegraph systems don't carry much power do they? I can see this working for communications quite feasably, although in the wild west, the dry ground would probably be quite resistant, so a long pole driven down to moist soild must have been used.

BTW, if you were to touch a live mains in the upstairs of a building is there any chance of getting shocked? I mean you have a huge resistance to earth from there, I can't imagine 240v producing any current through about 2 metres of air and other non-conductive materials? For that matter, my kitchen floor is vinyl for instance, the resistance between the sides of one tile must be many Mohms, surely I can't carry a significant current to earth stading on them?

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Obviously it depends on the materials - but bear in mind you're probably wearing rubber soled shoes as well, giving no path to ground. As I've said many times, you have to work really hard to get electrocuted - or be VERY unlucky, but people do manage it every year, even on 110V mains!.

For that reason we use rubber matting on the floor around the work benches.

Ah, so that is right about rubber. Considering that a carpet and it's underlay have nylon and rubber construction, a house foundation is thick concrete, some floors are laminated wood, I can see it must be nearly impossible as none of those things can conduct significant current at 240v. The only sure way I can see is standing barefoot on wet earth (hmm, this reminds me of mythbusters, peeing on train line). Of course contacting both live and neutral at once would be fatal if applied across the heart, but that isn't too easy to do by accident.

You're forgetting all the dangerous earthed objects about! - hand on radiator, other hand on live wire - BANG!.

Also damp conditions are FAR worse, hence why it's illegal in the UK to have electrical sockets in bathrooms (with the exception of transformer isolated shaver sockets), and why light switches are string-pull operated.

Now thats interesting. I've got a HV source here connected up to my currently earthed power supply. The source is 9v battery powered, but it is connected to a HV meter i've made (working rather nicely too, 140Mohm input impedance). Since the meter runs from the earthed supply, when i'm reading voltages, and touch any HV or otherwise live part of the HV source, I can feel a definate tingle in my finger from it. Disconnecting the battery powered HV source from the earthed meter stops this happening. Somehow current is flowing through me from earth, despite my being upstairs!?